Method and apparatus for plasma generation

ABSTRACT

An RF-driven plasma source, including a pair of spaced-apart plasma electrodes, wherein the electrodes act as plates of a capacitor, the gas electrically discharges and creates a plasma of both positive and negative ions, in a clean process that enables efficient sample analysis, with preferred isolated sample photo-ionization, reduced-power operation and also including signal detection with modulated drive electronics.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/471,854 (filed 21 Jun. 2006), which is a continuation of U.S. patentapplication Ser. No. 10/894,861 (filed 19 Jul. 2004), which is acontinuation-in-part of U.S. patent application Ser. No. 10/215,251(filed 7 Aug. 2002), the entire contents of which are incorporatedherein by reference, which claims priority to and the benefit of thefollowing applications: U.S. Provisional Patent Application 60/310,902(filed 8 Aug. 2001); U.S. Provisional Patent Application 60/335,219(filed 25 Oct. 2001); U.S. Provisional Patent Application 60/340,815(filed 12 Dec. 2001); and U.S. Provisional Application 60/388,052 (filed12 Jun. 2002). The U.S. patent application Ser. No. 10/894,861 alsoclaims priority to and the benefit of the following applications: U.S.Provisional Patent Application 60/488,019 (filed 17 Jul. 2003); U.S.Provisional Application 60/498,163 (filed 27 Aug. 2003); U.S.Provisional Patent Application 60/498,093 (filed 27 Aug. 2003); U.S.Provisional Patent Application 60/518,367 (filed 8 Nov. 2003); and U.S.Provisional Patent Application 60/520,284 (filed 14 Nov. 2003).

This application also incorporates herein by reference the entirecontents of U.S. patent application Ser. No. 10/462,206 (filed 13 Jun.2003).

FIELD OF THE INVENTION

The invention relates to an ionization source, and more particularly, inone embodiment, to a plasma generator for atmospheric gas dischargeionization.

BACKGROUND OF THE INVENTION

Creation of ionized particles is a useful tool for many applications,such as for ignition of lasing or to assist chemical analysis, amongother uses. In some equipment, high energy radioactive sources of alphaor beta particles are employed for the ionization process. However,because of the potential health hazard and need for regulation,wide-spread use of equipment using radioactive ionization sources hasbeen limited. And even though smoke alarms use radioactive sources, theamount of ionization is low, and they still require governmentregulation.

There are several ionization methods that avoid radioactive sources.Corona discharge is a source of non-radioactive ionization. It provideshigh energy in a compact package. However, this process is not stableand can contaminate the sample with metal ions or NOx, as wouldinterfere with analytical results. Furthermore, the generated ionspecies depends upon the applied voltage.

RF discharge ionization reduces some of these disadvantageous effects.RF discharges are subdivided into inductive and capacitive discharges,differing in the way the discharge is produced.

Inductive methods are based on electromagnetic induction so that thecreated electric field is a vortex field with closed lines of force.Inductive methods are used for high-power discharges, such as forproduction of refractory materials, abrasive powders, and the like.

Capacitive discharge methods are used to maintain RF discharges atmoderate pressures p˜1-100 Torr and at low pressures p˜10⁻³-1 Torr. Theplasma in them is weakly ionized and non-equilibrium, like that of acorona discharge. Moderate-pressure discharges have found application inlaser technology to excite CO₂ lasers, while low-pressure discharges areused for ion treatment of materials and in other plasma technologies.

Another ionization process is UV ionization. This process is sometimesreferred to as atmospheric pressure photo-ionization (APPI). In lowpressure conditions, photo-ionization involves direct interaction ofphotons with samples, forming positively charged molecular ions and freeelectrons. At elevated pressure conditions, the situation is not sosimple and the ionization process for sample molecules can include asequence of gas phase reactions, the details of which depend on theenergetic properties of initially formed ions and free electrons (due todirect photo-ionization) and on the nature of the ambient gas.

One disadvantage of UV ionization is that it provides low to moderateionization energies. This limits the types of molecules that can beionized. As well, sometimes APPI can give unexpected results. Thephotons are typically generated in a tube, with the photons passingthrough a window, and this window material affects efficiency. Also, thesurfaces of the UV devices can become contaminated or coated from theionization product, which can degrade device performance or outputintensity. As well, the UV tubes can be delicate and fragile, and henceare generally not suitable to operation in harsh environments or inapplications requiring a significant amount of manual handling.

SUMMARY OF THE INVENTION

The invention, in various embodiments, addresses the deficiencies in theprior art by providing reliable non-radioactive ionization sources forvarious applications. More particularly, in one aspect, the inventionprovides a capacitive discharge apparatus for generating a stable plasmaat pressures including at or around atmospheric pressure.

According to one embodiment, a plasma ionization source of theinvention, also referred to as a plasma generator or a plasma ionizer,includes at least two plasma electrodes spaced by an ionization gap. Inone practice of the invention, a system includes a capacitive gasdischarge plasma generator for generating a plasma and a sample ionizerfor ionizing a sample, with the sample ionizer being enabled by theplasma. In one embodiment, the gas is air and the plasma is formed atsubstantially atmospheric pressure and generates positive and negativeions substantially concurrently.

According to one advantage, the invention reduces or eliminates creationof ions from electrode material, and/or creation of other by-products,which may contaminate plasma ionization and which may impact furtherprocesses, such as sample ionization and analysis.

In various practices of the invention, the electrodes may or may not beprotected from the plasma. In some embodiments, the electrodes areoutside of the plasma environment or are otherwise isolated from theplasma to achieve a clean and more stable plasma. This favorably impactsdownstream sample analysis.

In various practices of the invention, plasma formation may be immediateto sample ionization or may be physically separated from sampleionization. Sample ionization may or may not occur in the plasma.Separation of plasma formation from sample ionization results incleaner, more reliable, and more stable sample ionization. This alsofavorably impacts downstream sample analysis.

In one practice of the invention, the plasma is formed in a gas flowchannel and at least one of the plasma electrodes is protected fromcontact with the plasma. One or more of the plasma electrodes may belocated external to the gas flow channel. Alternatively, at least one ofthe plasma electrodes may have an associated material layer forprotecting the surface of the electrode(s) from destructive contact withthe plasma. In one embodiment of the invention, the material layerincludes a low or non-conductive material (e.g., an insulator ordielectric) for isolating one or more of the plasma electrodes. In oneimplementation, the invention employs a dielectric of high permittivitymaterial to enlarge spacing between the metal electrodes of the plasmagenerator, while still achieving tight, effective gap spacing. Thisembodiment achieves a plasma with well-defined, temp-controlled emissionqualities.

In various implementations, electrode surface protection reduces and/orprevents erosion of electrode surfaces, and/or limits and/or preventsion contamination of the plasma. Thus, according to one advantage, aplasma generator of the invention is able to ionize a wide range ofcompounds for practical analytical applications, without contaminatingthe ionized sample.

In various practices of the invention, plasma formation results inionized atoms and molecules, generation of free electrons, and the like.This process may be favorably controlled by changing the plasma electricfield drive parameters. Regulation of drive waveform characteristics mayinclude selectively adjusting the plasma drive signal frequency and/ormagnitude and/or duty cycle, and/or by making changes in pressure,humidity, gas content, volume, and the like. According to anotheradvantage, the invention enables achieving controlled and selectiveionization and/or intended fragmentation of a sample, with control offormation of by-products (e.g., NOx), and/or control of undesiredformation of clusters, fragmentation, and the like.

According to various practices, the invention enables operation at awide range of ionization levels, as needed, from low to moderate to highenergy, and from “soft” to “hard” ionization, as desired.

Soft ionization involves charge attraction and transfer reactions andproduces molecular ions, and is non-destructive. Hard ionization resultsfrom electron impact and produces fragment ions. Both types ofionization may be achieved in practice of the invention. According toone feature, soft ionization may be selected for analysis of intactionized molecules and hard ionization may be selected for samplefragmentation for generating additional useful data, such as, withoutlimitation, for analyzing when complex mixtures. According to anotherfeature, the invention controls plasma intensity and ionization levelsas needed.

According to a further embodiment of the invention, one or more dopantsmay be introduced into the plasma process to change characteristics(e.g., frequency) of the emitted light, such as where a target photonoutput is sought, or to suppress interferences. Furthermore,introduction and control of dopants (e.g., acetone, water vapor, orother suitable dopant.) can reduce ignition energy or keep alive energy,as such function is expressed in known Paschen curves, for a givenplasma, as well as impacting species creation, density and energy.Control of plasma energy impacts rate, energy and efficiency ofionization in the plasma field.

Thus, according to one aspect, the invention enables control of aplurality of levels of ionization. Plasma ionization control improvessample ionization control. Such controls, in turn, improve the abilityto direct and control sample analysis.

According to a further embodiment, the invention provides substantiallyconcurrent, or in some embodiments, substantially simultaneousgeneration of both positive and negative ions in the plasma. Suchgeneration in the plasma enables a correspondingly similar generation ofpositive and negative sample ion species. In a one embodiment of theinvention, positive and negative sample ion species are generated andthen filtered, detected and identified substantially concurrently oreven simultaneously in a DMS system of the invention. One result isfast, reliable and efficient chemical analysis of ionized samplespecies.

In one practice, the invention includes a flow path for flow of a gasand a sample. Embodiments of the flow path enable local plasma formationand local sample ionization and, optionally, enable local sampleanalysis. The flow path may accommodate, for example, multiple flows,split flows and/or counter flows, and may include an analytical systemwith a plurality of flow channels for processing an ionized sample.

Ionizing the gas generates ionization media and by-products. In aparticular practice of the invention, a flow arrangement enables (1)flow of the by-products in the plasma field in a direction away from thesample ionizer, and (2) flow of the ionization media out of the regionof the plasma field and into the sample ionizer. Therefore, the sampleis advantageously ionized in the sample ionizer by the ionization mediaoutside of the plasma and away from the by-products. One result iscleaner and more reliable sample ionization.

In another practice, the plasma generator generates photons. In oneconfiguration, the sample is ionized with the photons outside of theplasma generator in a “windowless” photo-ionization arrangement. In oneembodiment, the invention includes a windowless atmospheric pressurephoto-ionization (APPI) system in which capacitive gas discharge plasmaionization is used as a source of photons, and the photons are useddownstream to ionize the sample outside of the plasma region.

The photo-ionized sample is then transported downstream for other use.Such arrangement improves ion species generation and analysis byavoiding the affect of the complex chemistry of plasma ionization uponsample processing. The windowless photon source is an improvement over atypical UV photo-ionization source and avoids inconsistencies andabsorption limitations that often stem from transmission through a UVwindow. This process reduces sample contamination and calibration needs.

Various electrode configurations are within the spirit and scope of theinvention, including planar, cylindrical, curved, molded, wire, and/orneedle shapes, which present any variety of flat, pointed, or curvedsurfaces and may be integrated into planar, curved, cylindrical and/orother suitably configured filters, separators and/or spectrometers, andmay be parallel or at an angle to each other. The gas sample may flow,for example, over, between and/or around the electrodes.

The flow path may have one or more inlets, accommodating one or moreflows, which may include gas, a sample, dopant(s) and/or other flows. Inone embodiment, multiple gas flows are arranged to permit selectiveintroduction of a selected gas, such as a dopant, to control, optimizeor stabilize plasma parameters. In another embodiment, a multiple flowchannel system is provided for localization of plasma formation, sampleionization, and sample analysis. In a further multi-channel device ofthe invention, multiple processes are performed for characterizing ionspecies. This arrangement may include a parallel or serial array ofprocesses.

Preferably the system includes a controller and the system is operatedfor optimization of plasma generation and sample processing.

In one practice, the flow path receives a dopant, the dopant flowing inthe plasma generator to affect the plasma generation. In anotherpractice, the dopant flows in the sample ionizer to affect sampleionization. In another practice, the dopant flows in the sample analyzerto affect analyzing of the ionized sample.

In another practice, a first flow channel part includes a flow inlet andoutlet and sample flow into the flow channel part and both positive andnegative sample ions are generated. The sample ions are transportedalong the first flow channel part toward the outlet and are influencedby an ion deflector, wherein selected ions of the positive and negativesample ions are deflected from the first flow channel part into thesecond flow channel part by the ion deflector. The selected anddeflected sample ions are processed in the second flow channel part bythe sample analyzer. The sample analyzer characterizes the sample basedon the processing in the second flow channel part of the selected anddeflected sample ions.

Efficient plasma generation may be implemented in circuit design. In onepractice of the invention, a resonant drive is used for generating theplasma, which produces a high frequency RF field at low power. In afurther practice, a feedback loop including a photo-detector isimplemented as part of a plasma drive stabilization circuit, resultingin a stabilized plasma ionization source. In various embodiments of theinvention such feedback may be implemented using a simple detector togauge photon intensity or a photo-spectrometer that evaluates photoemission spectra to more completely evaluate the plasma formation andsample ionization process.

In one practice of the invention, a resonant plasma drive circuitgenerates plasma formation. In a one embodiment, the plasma electrodesare driven with a modulated drive signal. This modulation stimulates theelectrodes to produce ions in “packets” having a frequency related tothe modulated drive signal, and having an average intensity controlledby the modulated drive signal. A system using this modulated drivesignal technique uses low power, provides precise and linear control ofplasma intensity. Plasma intensity sufficient to produce ion levelscompatible with a compatible sensor can be achieved.

Modulation of the plasma generator drive signal reduces plasma drivepower consumption. Use of such plasma modulation also improvesspectrometer performance in a preferred DMS embodiment of the invention.According to one feature, the plasma drive modulation encodes theionized sample signal to be detected. According to another feature, thischaracteristic of the modulation can be used to discriminate againstnoise contribution, which is spread over a different and wider band offrequencies.

Embodiments of the invention may also include improvements in DMS drivetechniques. Driving a DMS RF filter with a modulated RF drive signalimproves DMS operating efficiency. In one embodiment, a circuit drivesthe DMS filter electrodes to generate an asymmetric high RF field with aduty cycle and DC offset compensation voltage to filter the ions in theion flow by DMS techniques. A system using a modulated filter drivesignal technique may include a circuit operating with a low voltage DCsource (e.g., 20 volts) compared to traditional circuits that operatewith sources providing 200 Volts or more. As a result of the lowersource voltage, a low cost, small geometry MOSFET transistor can be usedin this circuit, and low cost components overall can be used.

According to some embodiments, the invention includes both plasmaionization methods and DMS filtering methods. According to otherembodiments, the invention includes both modulated plasma formation andmodulated DMS filtering methods.

Additional features, benefits and advantages of the invention arefurther described with respect to the following illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will now be described with respect to the accompanyingdrawings in which like reference designations refer to like partsthroughout the different drawings. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principles ofthe invention.

FIG. 1A is a generalized block diagram of a system employing acapacitive gas discharge plasma ionizer according to an illustrativeembodiment of the invention.

FIG. 1B is a more detailed block diagram of a system employing acapacitive discharge plasma ionizer according to an illustrativeembodiment of the invention.

FIG. 1C is an illustrative resonant RF drive circuit of the type thatmay be employed in the system of FIG. 1B.

FIG. 2A is a graph depicting positive and negative spectra generatedwith a radioactive source.

FIG. 2B is a graph depicting positive and negative DMS spectra generatedby a capacitive according to an illustrative embodiment of the inventionand having both electrodes insulated.

FIG. 3A is a graph depicting positive and negative spectra generatedwith a radioactive source having one electrode insulated.

FIG. 3B is a graph depicting positive and negative DMS spectra generatedby a capacitive discharge plasma ionization source according to anillustrative embodiment of the invention and having one electrodeinsulated.

FIGS. 4A and 4B are graphs showing a comparison of the mass positivespectra (FIG. 4A) from a radioactive source and (FIG. 4B) from anillustrative embodiment of the invention, as detected by a massspectrometer with a low plenum gas flow.

FIGS. 5A and 5B are graphs showing negative mode mass spectrometerspectra for (FIG. 5A) air, and (FIG. 5B) air plus 20 ppm of SF₆ (M=146),after plasma ionization according to an illustrative embodiment of theinvention.

FIG. 6 is a graph showing DMS detection of mercaptan in purified airionized in a capacitive gas discharge plasma ionizer according to anillustrative embodiment of the invention.

FIG. 7 is a graph showing mass spectra for acetone generated andreproduced by ionization of acetone according to an illustrativeembodiment of the invention.

FIG. 8 is a diagram of a capacitive discharge plasma ionizer structureaccording to an illustrative embodiment of the invention.

FIGS. 9-12 are diagrams of capacitive discharge plasma ionizerstructures according to alternative illustrative embodiments of theinvention.

FIGS. 13A-13J show diagrams of alternative electrode configurations fora capacitive discharge plasma ionizer according to various illustrativeembodiments of the invention.

FIGS. 14A-14C show additional illustrative electrode configurations fora capacitive discharge plasma ionizer of the invention.

FIGS. 15A-15F show further illustrative electrode configurations for acapacitive discharge plasma ionizer of the invention.

FIGS. 16A-16D show additional illustrative planar, non-planar andcontoured electrode configurations for a capacitive discharge plasmaionizer according to the invention.

FIG. 17 shows a diagram of intermeshed electrodes for a plasma generatorformed on a single substrate according to an illustrative embodiment ofthe invention.

FIG. 18 shows a diagram of intermeshed electrodes for a plasma generatorformed on a single substrate and rotated 90 degrees on the surface ofthe substrate according to another illustrative embodiment of theinvention.

FIG. 19 shows a diagram of a DMS microchip device with opposedsubstrates and having a plurality of electrodes for plasma ionizationand sample analysis according to an illustrative embodiment of theinvention.

FIGS. 20A-20I are diagrams of illustrative flow structures exemplary ofthe type that may be employed with capacitive discharge plasma ionizersaccording to various illustration embodiment of the invention.

FIG. 21 is a diagram of an illustrative DMS system having a plasmaphoto-ionization source and split gas flow according to an illustrativeembodiment of the invention.

FIG. 22 is a diagram of an alternative illustrative embodiment of theanalyzer and plasma photo-ionization source of FIG. 21.

FIG. 23 is a block diagram of a system including a differential mobilityspectrometer connected to an ionization source employing the principlesof the an illustrative embodiment of the invention.

FIG. 24 is a graph of optical intensity output versus voltage for aconventional plasma generation source.

FIG. 25 is a graph of optical intensity versus duty cycle for a plasmageneration source using a drive circuit according to the principles ofan illustrative embodiment invention.

FIGS. 26A-26C show graphs of illustrative waveforms for driving a plasmagenerator of the invention.

FIG. 27 is a time plot of a modulated drive signal having a duty cycleused to produce an output consistent with the graph of FIG. 25 accordingto principles of an illustrative embodiment of the invention.

FIG. 28 is a block diagram showing ions produced in the system of FIG.23 in response to the drive signal of FIG. 27, and also including aschematic diagram of a detector circuit to measure the ions.

FIG. 29 is a flow diagram of an illustrative process for measure theions of FIG. 28.

FIG. 30 is a timing diagram indicating an illustrative time of flightfor the ions of FIG. 28.

FIG. 31 is a generalized schematic diagram of an illustrative filterdriver for the system of FIG. 23.

FIG. 32 is a generalized schematic diagram depicting parasiticcapacitance and leakage inductance associated with the transformers ofthe circuit of FIG. 31.

FIG. 33 is an a graph of an RF waveform exemplary of the type producedby the circuit of FIG. 31.

FIG. 34 is a graph of an exemplary low frequency, compensation voltagewaveform for modulating the RF waveform of FIG. 33.

FIG. 35 is a diagram of a filter and detector of a type that may beemployed in the system of FIG. 23 and having a compensation voltage ofthe type depicted in FIG. 34 applied to the filter.

FIG. 36A is a top view of a circuit board trace used to implement aprimary or secondary winding in the transformers of FIG. 31.

FIG. 36B is a cross-sectional view of a circuit board layering diagramhaving indications of traces used to implement the primary and secondarywindings of the transformers of FIG. 31.

FIG. 37 is a top view of a circuit board trace used to implement aprimary or secondary winding in the transformers of FIG. 31 according toan alternative illustrative embodiment of the invention.

FIG. 38 is a mechanical diagram of a transformer of FIG. 31 implementedin a circuit board using the techniques discussed with respect to FIGS.35-37.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The invention, in one aspect, relates to plasma-assisted sampleionization. According to one embodiment, the invention featurescapacitive discharge plasma ionization of a chemical sample and ananalyzer for analyzing the ionized sample. Turning to the illustrativeembodiments of FIG. 1A-1C, the system 10 includes a capacitive dischargeplasma ionization source 11 and an analytical system 20, wherein thesample S and the carrier gas CG flow into a plasma field F in the plasmasource 11. The ions M+ and M− from the ionization process flow along theflow path 12 out of the plasma source 11 and into the analyzer 20 foranalysis.

In the illustrative embodiment, the analyzer 20 provides an analyticalelectric field for analyzing ions associated with the ionized sample.The analyzer 20 may be, for example, a mass spectrometer, an IMSspectrometer or other suitable detector. According to one illustrativeembodiment, the analyzer 20 is DMS spectrometer and the analysis isbased on aspects of the mobility of the ions in an analytical field.Detection of ion species of interest is indicated as m+/− output fromthe analyzer 20.

In FIG. 1B, the plasma source 11, also referred to as a plasma generatoror plasma ionizer, is separate from, but in communication with, theanalyzer 20. Alternatively, the plasma source 11 may be integrated intothe analyzer 20 to form an integrated system 10.

The capacitive discharge plasma ionization source 11 includes a pair ofelectrodes 14 and 16, which are preferably insulated from gas reactionsin the plasma ionization region 36. In response to a sufficient drivevoltage being supplied across the electrodes 14 and 16, a dischargefield F is established. According to the illustrative embodiment, theplasma drive voltage ranges from about 1 to about 100 kHz and ismodulated in some aspect (such as in intensity, duty cycle, frequency,or the like).

In embodiment of FIG. 1B, a carrier gas CG (also referred to as atransport gas) and sample S are fed through an inlet 13 into a plasmaionization region 36. The transport gas is ionized by capacitivedischarge between the electrodes 14 and 16. This discharge processproduces a plasma 40, which ionizes the gas CG and the sample S withboth positive and negative ions, M⁺, MH⁺, and M⁻, and illustrativelygenerates (H20)_(n), H⁺, O⁻, O₂ ⁻, O₃ ⁻, (N_(x)O_(n))⁺ and/or(N_(x)O_(y))⁻(H₂O)_(n).

The generated ions in the ionization region 36 exit through a passage 37for further downstream utilization. In an analytical embodiment of theinvention, these ions proceed from the passage 37 into the spectrometer20 for analysis, as shown in FIG. 1B.

FIG. 1B shows an illustrative control and drive circuit 22. The controland drive circuit 22 is depicted in more detail in FIG. 1C. As shown,the illustrative circuit 22 includes a pulse generator 22 a, a resonancegenerator 22 b, and a resonant circuit 22 c. The resonant circuit 22 cincludes the electrodes 14 and 16 (spaced by an ionization gap G) and aninductor L. A microchip or other logic or controller device 22 d mayalso be supplied in communication with drive circuit 22, and optionallymay include inputs from other system feedback or data sources, to affecttotal system control. The control and drive circuit 22 may be drivenusing known techniques. The control and drive circuit 22 may also employan optimization routine for selecting operating conditions based on theabove mentioned system inputs.

Plasma sources of the invention offer a viable alternative toradioactive ionization sources. FIG. 2A shows positive and negative DMSspectra generated with a radioactive ionization source (⁶³Ni at 10 mCu)and FIG. B shows positive and negative DMS spectra generated by acapacitive discharge plasma ionization source, such as the source 11,with both electrodes 14 and 16 being electrically isolated from eachother (insulated) and/or physically isolated from the plasma (forexample, by applying a low or non-conductive coating, such as adielectric or an insulating material, to the electrodes), according toan illustrative embodiment of the invention. This comparisondemonstrates that the non-radioactive ionization source 11 of theinvention can replace a radioactive source and provide similarperformance.

FIG. 3A shows positive and negative DMS spectra generated with theradioactive ionization source of FIG. 2A, but with only one ofelectrodes 14 and 16 being electrically and/or physically isolated, forexample, by application of a coating as described in relation to FIG.2A. FIG. 3B shows positive and negative DMS spectra generated plasmagenerator 11, also with only one of electrodes 14 and 16 being isolated(electrically and/or physical, as described above). As can be seen, thepositive spectra from the radioactive and the plasma sources are nearlyidentical. However, the negative spectra in FIG. 3B, with one electrodeprotected (for example, by being isolated as a result of an applicationof a low- or non-conductive coating)—the protection being from, forexample, accelerated ions and/or electrons—is somewhat degraded versusthat of FIG. 3A, where both electrodes are protected.

In implementations of various illustrative embodiments, we have foundthat the plasma source of the invention is capable of providing adequateionization energy in many applications, operating on as low as about afew watts or lower (e.g., about 0.5 watts in one embodiment). We alsoobserved that in the comparisons described with respect to FIGS. 2A-2Band FIGS. 3A-3B, the beta source was capable of generating a maximum ioncurrent of about 4 pA, while the above described illustrative embodimentof the invention delivered a maximum of about 12 pA. Thus, one importantadvantage of the invention is that it provides an efficient and powerfulplasma ionization source.

FIGS. 4A and 4B show graphs depicting a comparison between the positivespectra from a ⁶³Ni radioactive source (FIG. 4A) and from a plasmasource (FIG. 4B) of the invention. The spectra of FIGS. 4A and 4B were,detected by a mass spectrometer with a low plenum gas flow (i.e., abarrier counter-flow of clean gas to prevent introduction of laboratoryair into the MS). The system of FIG. 4A was capable of generating about4,000 ions per second, while the system of the invention (FIG. 4B)recreated the same or comparable spectra, while achieving an ionproduction rate of about 50,000 ions per second. Thus, another importantadvantage of the invention is that it can provide a rich source of ionsfor a broad range of applications. It is further noted that while FIGS.4A-4B show MS results with a low plenum gas flow, the invention is notlimited to particular flow rates, whether in the plasma ionizer (sampleand carrier gas) or in a DMS analyzer (ion flow) or at the front end ofan MS (plenum).

FIGS. 5A-5B show negative mode mass spectrometer spectra for air and airplus 20 ppm of SF₆, respectively, after plasma ionization, according toan illustrative embodiment of the invention. Comparing the two frames,the SF₆ (M=146 amu) peak stands out and is clearly identified, while thebackground spectra retains its integrity.

Exceptional detection results may also be obtained using other detectiondevices. For example, in FIG. 6, a DMS spectrometer received an ionizedoutput of a mercaptan sample and purified air as outputted by an acapacitive gas discharge plasma generator of the invention. As can beseen from the graph of FIG. 6, the negative and positive mercaptan (+/−mer.) peaks and background spectra are clearly defined. FIG. 7 shows agraph depicting another illustrative application of the invention inwhich the mass spectra for acetone was generated and reproduced by softionization of the acetone.

FIG. 8 depicts a system 10 including a capacitive discharge plasmaionization source 11 positioned within a flow channel 12 according to anillustrative embodiment of the invention. The ionization source 11defines an ionization region 36 about the ionization source 11. The flowchannel 12 has a planar geometry formed by upper and lower substrates 24and 26. Alternatively, the ionization source 11 may be placed within acylindrical, polygonal (e.g., rectangular), or otherwise suitably shapedflow channel. According to various illustrative embodiments, the flowchannel may include flat or curve surfaces, and the surfaces may berelatively smooth or textured.

The ionization source 11 of FIG. 8 includes a first electrode 14 placedwithin an isolating capillary tube 18 and a second electrode 16 wrappedaround the capillary tube 18. The electrodes 14 and 16 are separated bya gap G. One end of each of electrodes 14 and 16 is connected to the RFdrive voltage supply 22 such that the electrodes 14 and 16 function asthe plates of a capacitor, with the drive RF voltage applied acrossthem.

FIG. 9 shows another illustrative embodiment of the invention, in whicha plasma ionization source 11 includes an isolation substrate 18. Asshown in FIG. 9, the isolation substrate may include, for example, aglass capillary tube. The isolation substrate 14 may be coated with ametallization layer 23. The metallization layer 23 is parted at “x” todefine two metallization regions forming the electrodes 14 and 16. Inone illustrative configuration, the inner faces 14′ and 16′ of theelectrodes 14 and 16, respectively, are formed on the isolationsubstrate surface of the tube 18 and face each other through thecapillary tube 18 across the open lumen 18 c. An RF signal from avoltage source, such as the voltage source 22, may be applied to theelectrodes 14 and 14 of FIG. 8 to generate a field F within the lumen 18c of the capillary tube 18.

In the illustrative embodiment of FIG. 9, the gap separating theelectrodes 14 and 16 is defined by the outside diameter of the capillarytube 18. Within the tube 18, the entire lumen 18 c may be utilized as anionization region. In one operation, the gas CG and sample S are flowedinto the lumen 18 c of the capillary tube 18 through the inlet 13. Thegas CG is ionized and forms a plasma field F, which in turn ionizes thesample S between the electrodes 14 and 16. This process generates bothpositive and negative ions, and ionizes the sample into both positiveand negative ions +/−, which can be isolated. The ions subsequently exitthrough outlet 15 for further use, such as in an ion mobilityspectrometer, and in one preferred embodiment, in a DMS that processesboth positive and negative species simultaneously.

FIGS. 10 and 11 show alternative illustrative embodiments of theinvention in which the conducting electrodes 14 and 16 are placed intoadjacent tube-like dielectric sheaths 86 and 88, formed from, forexample, glass, quartz, ceramic or other suitable material. Preferably,the dielectric sheaths 86 and 88 are fixtured so that the separationbetween the electrodes 14 and 16 is fixed within the ionization region36. This separation can range, for example, from having the dielectricsheaths 86 and 88 touching to having a separation of about 5 mm or more.

As shown in FIG. 10, the electrodes 14 and 16 may be held and joined viacollars 92 and 94. Just beyond the collar 94, the ionization region iseffectively terminated after the electrodes 14 and 14 diverge. Thisarrangement enables defining the length of the ionization region andthus provides predictable performance characteristics. The abuttingcollars 96 and 98 are affixed on each of the tubes 86 and 88 after thecollar 94 to fix this divergence. In various illustrative embodiments,the electrodes 14 and 16 may be formed of conventional thin wirefilaments and may be contained in a tube or coated with a dielectric orother insulating material.

FIG. 12 shows an alternative illustrative embodiment of the inventionemploying diverging curved plasma electrodes 14 and 16. In thisembodiment, the field F is formed between the diverging electrodes 14and 16. In other illustrative embodiments, the plasma electrodes 14 and16 may be, for example, parallel or angled relative to each other, berelatively straight or curved, have relatively smooth or textured innerand out surfaces, or any combination of the above.

As described above, the electrodes 14 and 16 are separated by a gap,whether exposed or isolated, embedded in a dielectric material, orwithin isolating tubes, for example, and whether parallel or diverging.Additionally, the electrode diameter and isolation coating material typeand diameter/thickness may be selected such that the fields generatedbetween the electrodes 14 and 16 are accessible to the gas flow. In FIG.11 the gas flows between the electrodes 14 and 16 and therefore, throughthe plasma-generating field between the electrodes 14 and 16. However,in FIG. 10, the gas flows along the perimeter of the tubes. Preferably,the field generated between the electrodes extends into this perimeterflow. According to the illustrative embodiment, the drive signal 22, thefilaments 14 and 16, and the coating diameter are selected toaccommodate generation of the plasma ionization field F in thisperimeter area.

FIG. 13A shows a rectangular cross-section plasma flow arrangementwherein the gas is contained and flows between the plasma electrodes 14and 16 in the field F, according to an illustrative embodiment of theinvention. In the embodiment of FIG. 13B, the electrode 14 is on asurface of a first substrate 24, and the electrode 16 is on a surface ofa second substrate 26. The electrodes 14 and 16 are driven by thevoltage source 22.

An isolation layer 34 of, for example, Al₂O₃ (Alumina) or SiO₂, or othersuitable material, is formed over one or both of the electrodes 14 and16. In the embodiment of FIG. 13B, the ionization source 11 is arrangedwith the opposing surfaces of the inner isolators 34 being spaced apartby about 10 μm or more to define the plasma ionization region 36.

As discussed above, FIG. 12 shows curved diverging tube electrodes 14and 16 for plasma generation. In FIG. 13C, the electrodes 14 and 16 arepositioned at an angle to achieve a divergence. The electrodes 14 and 16are formed respectively on the upper and lower substrates 24 and 26. Theangle between the substrates 14 and 16 is selected such that theionization region 36 has a narrow region 41 a and a wide region 41 b. Ahigher field strength is created in the narrow region 41 a relative tothe wide region 41 b as a result of the electrodes 14 and 16 beingcloser together. This in turn creates a higher field strength and hencea more intense ionization field, which can assist in plasma ignition. Inthe illustrative embodiment of FIG. 13C, the gas that enters the plasmaionization region 36 is first ionized in the narrow region 41 a. Theelectric field travels from the narrow region 41 a to the wide region 41b and the ionization process propagates accordingly to generate the ions(++, −−).

In the embodiment shown in FIG. 13D, a pair of ionization sources 11having non-parallel electrodes cooperate to form a plasma generator,according to an illustrative embodiment of the invention. The ionizationsources 11 are positioned within a flow channel 110 defined by an uppersubstrate 100, a lower substrate 102, a first spacer plate 104, and asecond spacer plate 106. As the sample enters the ionization region 36,the plasma formation and ionization process initiates in the narrowregions 41 nearer each spacer plate and then progresses towards thewider regions 42 nearer the center of the channel 110.

Diverging electrodes are not required to create the above discussedintense ionization regions. FIG. 13E, which depicts an extension of theillustrative embodiment of FIG. 13D can also provide similar intenseionization regions. Referring to FIG. 13E, the ionization source 11includes a first electrode 114 mounted to an upper substrate 100 and asecond electrode 116 mounted to an inner surface of a lower substrate102. In addition, a third electrode 118 and a fourth electrode 120 aremounted to the inner surfaces of side spacer substrates, 104 and 106,respectively.

The electrodes 114, 116, 118, and 120 couple to a drive control/sourceand are arranged such that the electrodes 114 and 118 form one capacitorand the electrodes 116 and 120 form another capacitor. Two of theelectrodes (e.g., 114 and 118) are of the same polarity, while the twoothers (e.g., 116 and 120) are of the opposite polarity, such that thereare four intense plasma formation and ionization regions 140, 142, 144,and 146 near the corners of the electrodes. When the gas enters theplasma ionization region 36, the ionization process begins at theseintense ionization regions and then propagates toward the center 150 ofthe ionization region 36.

The plasma electrodes can operate without an isolator or at leastwithout an insulator on both electrodes. For example, in an alternativeembodiment shown in FIG. 13F, the electrode 16 is not covered by anisolating material. Furthermore, the electrode 14 need not be covered bythe isolator. That is, both electrodes 14 and 16 may be exposed directlyto the sample gas.

Alternatively, as shown in FIG. 13G, the electrodes 14 and 16 may bemounted to the respective outer surfaces of the substrates 24 and 26,need not be covered by an isolating material. In another alternativeembodiment, one electrode is within a favorable environment, e.g.,encapsulated in dielectric, while the other electrode is adjacent to thecarrier gas flow to be ionized in the generated plasma field. Asillustrated in FIG. 13H, the electrode 16 is mounted to the substrate26, wherein the electrode 14 includes a metal layer 52 a on one side ofa dielectric substrate 50. The opposite side of the dielectric substrate50 can also be coated with an additional metal layer 52 b. In eithercase, the electrode 14 is attached to the substrate 24 by suitableattachment mechanism, such as, for example, epoxy glue.

Referring now to FIG. 13I, an alternative embodiment of the ionizationdevice 10 includes an accelerator electrode 60, having its own selfpotential, mounted to the isolator layer 34 which covers the electrode14. Alternatively, the electrode 14 and the accelerator electrode 60 maybe mounted on opposite sides of the substrate 24, as depicted in FIG.13J. The accelerator electrode 60 advantageously enables additionalcontrol of the ion flow.

As shown in FIG. 14A, the accelerator electrode 60 may include a seriesof small electrodes 62 interconnected with conductive wires 64. Inanother illustrative embodiment, as shown in FIG. 14B, the acceleratorelectrode 60 may include a mesh of interconnected horizontal 66 andvertical 68 wires. Alternatively, as shown in FIG. 14C, the acceleratorelectrode 60 may include an ensemble of small conductive electrodes 61that are surrounded by a ring of conductive material 63, such as, forexample, a conductive metal.

As discussed above, plasma generators of the invention may be formedwith a variety of electrode configurations, which need not be planarplates. As shown in FIGS. 15A-15D, a needle electrode 170 may be used inconjunction with a planar electrode 174 or another needle electrode 170.In FIG. 15A, a needle electrode 170 is coated with an insulator 172 andcooperates with a planar electrode 174 to form a plasma generator of theinvention. In FIG. 15B, the planar electrode 174 is coated. In FIG. 15Cboth are coated, and in FIG. 15D the planar electrode 174 is replacedwith a second needle electrode 170, which may have coating 172.

FIG. 15E shows a plasma generator having parallel electrode faces 14″and 16″ on a single substrate 24. FIG. 15F shows the divergingembodiment of FIG. 13C configured on a single substrate 24 withdiverging electrode faces 14″ and 16″.

FIGS. 16A-16C show additional planar, non-planar and contoured plasmaelectrode configurations, respectively, according to variousillustrative embodiments of the invention. In some illustrativeembodiments, the invention employs a dielectric 34 (such as a floatingdielectric as shown in FIG. 16A), which may be, for example, of highpermittivity material, such as ceramic, or ink, and enables largerspacing between the metal electrodes, while still achieving a tight,effective gap G spacing. Such a configuration achieves well defined,temp-controlled emission qualities in practice of embodiments of theinvention. In this illustration, an adhesive, such as glass frit 27,bonds the dielectric layers 34 and 35, electrodes 14 and 16, andsubstrate support structure 24 and 26.

FIG. 16D shows an application of the configuration of FIG. 16C, but withthe electrodes formed directly on the ceramic substrates 24 and 26,without the dielectric. In FIG. 16D, the plasma generator 11 is formedbetween two substrates 24 and 26. The substrates 24 and 26 include aplurality of electrodes, which extend above a local recessed area oneach substrate. More specifically, one or more of the electrodes 14 and16 extend above and beyond an adjacent substrate recess 24 r or 26 r,provided on each substrate 24 and 26, respectively. The recesses 24 rand 26 r and the associated edge features 24 e and 26 e provide ageometric field enhancement and have the effect of focusing the plasmaat or above the plasma electrodes 14 and 16 away from the substratesurfaces. The electrodes 14 and 16 may be formed, for example, asmetallization layers 14 m and 16 m on the insulating substrates 24 and26, respectively, above the associated recesses 24 r and 26 r and edgefeatures 24 e and 26 e.

In the illustrative embodiment of FIG. 16D, the plasma 40 is formed inthe plasma ionization region 36 somewhat distant from the supportingsubstrate surfaces. This in effect isolates the corrosive effects ofplasma interaction from the neighboring and recessed substrates 24 and26, with the effect of reducing the complexity of the plasma ionizationprocess. This approach makes it much less likely that matter fromdesorption, surface erosion and the like, will contaminate the ion flow.As a result, a more efficient, reliable and stable analytical ionizationsource is realized.

Turning now to FIG. 17, the electrodes 14 and 16 are formed on a singlesubstrate 24″ (such as discussed above with respect to the embodimentsof FIGS. 15E-15F). The electrodes 14 and 16 each extend to define anumber of tines, such as tines 14 a, 14 b, 14 c, 16 a, and 16 b. Thesetines enable the electrodes 14 and 16 to be intermeshed while formingthe plasma generator 11 on a single substrate. The electrodes 14 and 16are driven by the RF source 22. FIG. 18 depicts a similar embodiment,but with the tine orientation rotated ninety degrees.

As discussed above, in one preferred embodiment, the electrodes 14 and16 are isolated from the gas flow. In the embodiments of FIGS. 17 and18, an isolating layer, for example of a dielectric coating 34, such as,for example, Al₂O₃ (Alumina), SiO₂, or the like, preferably formed oneach exposed electrode (and tine) surface, provides the requisiteisolation. The isolation layer is indicated in FIG. 18 by a dottedoutline.

As shown in FIG. 19, the single substrate may be enclosed in a flowchannel defined by a housing M to provide an enclosed plasma generator11 of the invention, with a sample intake at an inlet 265 and an exhaustat an outlet 266. It should be noted that electrode isolation is notshown in FIG. 17 and FIG. 19, but may be similarly employed in thoseembodiments.

The plasma generator 11 of the invention may be formed on the samesubstrates that incorporate a DMS device. As shown in the illustrativeembodiment of FIG. 19, a microchip 290 may be formed incorporating aplasma generator 11 of the invention, with mating opposed substrates 224and 226 for the electrodes. In various embodiments, a separate plasmagenerator, such as that shown in FIG. 17, may be formed on each of thefacing substrates or. Alternatively, a single plasma generator may beformed. A DMS device 240, having a filter 250 and optionally a detector260 may also be defined within the same microchip structure 290.

In the analytical system 10 shown in FIG. 19, the carrier gas CG andsample S are introduced at inlet 265, and are ionized by the plasmaprocess at plasma generator 11. The ionized particles are analyzed inthe DMS device 240 (at the DMS filter 250). The filter 250 output may bedirected to the input of a mass spectrometer or other detector device orsimply to an onboard detector 260, as shown, as it flows toward theexhaust 266.

In various illustrative embodiments of the invention, separation of thesubstrates and accurate spacing of the electrodes is desirable and maybe achieved as needed, such as by use of spacer parts 292 and 294 in themicrochip structure 290 of FIG. 19. The substrates 224 and 226 areformed mated against the spacers 292 and 294, which may be integralextensions of the substrates, or a housing, or separate components, asneeded, and may form a sealed structure.

As can be seen from the above discussion, advantages of the inventioninclude that, in various illustrative embodiments, it provides alow-cost, non-radioactive, highly-efficient, clean and stable, radiofrequency plasma ion source for using in fluid flows. According to otheradvantages, illustrative embodiments of the invention are capable ofproviding a wide range of plasma levels and are operable at low powerover a range of pressures, including atmospheric pressure, in air orother gas environments. According to further advantages, illustrativeembodiments of the invention are capable of ionizing a wide range ofcompounds, ranging from those having low ionization potential (such asacetone) to those having high ionization potential (such as SF₆), amongvarious other compounds, for example.

Illustrative embodiments of the invention can also be operated with goodcontrol over formation of ions and ion species. As an illustration, theamount of energy in the plasma can be controlled, such as by control ofthe energy supplied by drive circuit 22. Control of the amount of energyimparted into the gas and the resulting plasma controls the ion speciesgenerated in the plasma. By enabling control this energy, the inventionprovides control of the formation of ions. This control may also beexercised to prevent formation of unwanted ions, such as nitrogen ions(NOx species), which can interfere with detection of other negativeions. This control can also be employed for increasing and/or decreasingfragmentation or clustering for a particular downstream use.

According to various illustrative embodiments, the ionization device 11is suitable for use in many types of gas analyzers and detectors. Forexample, FIG. 20A depicts a DMS system 10 having an ionization device 11upstream for plasma ionization. Ions are generated for chemical analysisof a sample S in a carrier gas CG.

More particularly, the system 10 of FIG. 20 includes an ionizationsource 11, an ion filter 72 in the filter region 74 defined betweenfilter electrodes 76 and 78, and a detector 80 in a detection region 82between detector electrodes 84 and 86. Asymmetric field and compensationbias signals or voltages are applied to the filter electrodes 76 and 78by a drive circuit 88 within a control unit 90. The detector electrodes84 and 86 are also under the direction of the drive circuit 88 and thecontrol unit 90.

Briefly, in operation, the carrier gas CG, is ionized in the plasmaregion 36 forming ions ++,−− and the sample S is ionized creating bothpositive and negative ions, M⁺ and M⁻. Based on DMS ion filteringtechniques, only certain ion species pass through the filter region 74,while others are filtered out (i.e., they are neutralized by contactwith the filter electrodes 76 and 78). Those that pass through aredetected at the detector electrodes 84, 86. Preferred DMS configurationsare described in greater detail in U.S. Pat. Nos. 6,495,823 and6,512,224, the entire contents of both of which are incorporated hereinby reference.

As depicted in FIG. 20A, the electrodes 14, 76 and 84 are coplanar andthe electrodes 16, 78 and 86 are coplanar, being formed on thesubstrates 24 and 26, respectively. Alternatively, as shown in FIG. 20B,the system 10 includes a necked down, reduced diameter/width region 99of the substrates in which the ionization source 11 and the electrodes14 and 16, reside. In this configuration, the electrodes 14 and 16 arespaced apart by less than the distance separating the filter electrodes76 and 78. This enables a closer orientation of the plasma generatingelectrodes 14 and 16, while a greater separation is provided for thefilter electrodes 76 and 78.

In FIG. 20B, the capacitive discharge ionization source 11 is integratedwith a DMS apparatus 240 in analytical system 10. The system 10 of FIG.20 B includes a filter 72 and detector 80 formed on the substrates 24and 26. In this embodiment, at least one and preferably two of theelectrodes 14 and 16 is protected from the plasma with a dielectric 34.The filter 72 applies a compensated high field asymmetric waveform to apair of filter electrodes 76 and 78, which generates a high electricfield there between. According to ion mobility characteristics of theions passed into the DMS filter field, a species of ions is passed fordetection at the detector electrodes 84 and 86 of the detector 80. In atypical DMS manner, the detection event is correlated with the applieddrive voltages and known device performances to characterize thedetected ion species. According to a feature of the invention, this canalso be correlated with drive and control of the ionization device 11,for total analytical system control.

Positive and negative ions are generated in the plasma generator andconsequently positive and negative sample ions are presented to the DMSfilter. It is a characteristic of preferred DMS systems of theinvention, including in-line plate-type DMS systems, that both positiveand negative ion species can be filtered substantially concurrently, andin some embodiments substantially simultaneously. Those speciesrequiring the same compensation will pass substantiallyconcurrently/substantially simultaneously to the detector. The dualelectrode detectors 84 and 86 then detect these passed speciessubstantially concurrently/substantially simultaneously.

In operation, the carrier gas, with a sample of chemical compounds, isinputted at the upstream inlet 95 and the gas flows through theapparatus and out exhaust outlet 97. Gas flow rate and pressure may becontrolled by use, for example, of a downstream exhaust pump 91. The DMSsystem is driven and controlled by controller and driver circuit 88,which may be incorporated into and packaged with the plasma controllerand drive circuit 90. The plasma generating electrodes 14 and 16, filterelectrodes 76 and 78, and the detector electrodes 84 and 86 may all beseparate and distinct structures or may be formed as electrodes on thesurfaces of the substrates 24 and 26, for example.

In another illustrative embodiment, shown in FIG. 20C, the ionizationsource 11 is located within a channel 101 in between the substrates 100and 102. In this arrangement, the carrier gas CG, splits and partlyflows within the ionization region 36, where it is ionized to form theplasma, ++,−−, and also outside of the ionization source 11. The sampleS flows into the plasma ions ++,−− within the ionization region 36 andis ionized, but at a higher concentration versus the reduced carriergas. These ions flow out of the ionization source 11 and are carrieddownstream for further analysis.

In the embodiment of FIG. 20C, the efficiency of ionization of thesample is increased by reducing the amount of carrier gas in theionization region. In an alternative embodiment of the invention, alsoshown in FIG. 20B, to increase the ratio of the sample S to the carriergas CG, and to increase the efficiency of ionization of the sample S,the sample S is introduced into the ionization source 11 with a reduced,and preferably minimized, amount of carrier gas CG. The carrier gas isionized within the plasma. The sample is also ionized. All of this flowsinto a flow mixing and sample ionization region 59, where a secondstream of carrier gas CG is introduced via an additional inlet 105(shown in dotted outline in FIG. 20B) to carry the ions downstreamtoward the filter 72 for further analysis. A feature of this embodimentis that a lower amount of background gas is ionized, which relativelyincreases the ratio of the ionized sample to the ionized carrier gas,thus reducing the effect of the reactant ion peak in the analysis andimproving the effective ionization chemistry.

Referring now to FIG. 20D, an alternative system of the invention 10includes plasma ionization device 11 which receives a carrier gas flowCG. The carrier gas is ionized and these ions ++,−− flow into anionization region 59 where sample S from the source 71 is ionized. Thesample S is delivered to the ionization region 59, such as by a flow ofcarrier gas from the source 71 after the plasma ions are generated. Thesample is ionized in the region 59 and the ionized sample molecules M+,M− are then carried downstream accordingly. This arrangement avoids thecomplex ionization chemistry that can occur when ionizing sample withinthe plasma ionization section 11, and also can reduce unwanted affectsof plasma ionization chemistry upon the sample ionization and downstreamanalysis

In a further embodiment, biased accelerator electrode(s), 73, such asshown in FIG. 20D, can be provided to assist delivery of selectedportions of the ionized sample into the DMS filter region 74. In anotherillustrative embodiment, shown in FIG. 20E, a counter flow 70 of carriergas and sample is carried into the ionization device 11 for ionization.These ions then flow downstream to the sample ionization region 59 forionization of the sample and generation of ions to be analyzed. Thecounter flow 70 sweeps the flow of unwanted ionization products awayfrom ionization region 59. Also, a variety of gases which are differentfrom the sample can be mixed to create ions, e.g., use of dopants suchas acetone, water, etc.

FIG. 20F shows a plural channel embodiment 10 of the invention,including a first flow channel 270 having a plasma ionizer 11, whichreceives the gas flow CG and generates the ions ++, −−. These ions flowinto an ion diverter, which may be a baffle or another gas flow, toinfluence and redirect the ion flow. Preferably, the diverter isselective. For example, FIG. 20F includes a diverter 271, which divertsselected ions into second flow channel 272 through opening 273. Theremaining flow and other by-products of the plasma ionization processare exhausted out of flow channel 270 at vent 280.

The diverter may also be a biased electrode. In one embodiment, as shownin FIG. 20F, diverter 271 includes first and second electrodes 274 and275 which are independently biasable. In one illustration, electrode 274is negatively biased which drives the negative ions −−,−− generated inthe plasma ionization part 11 through access 273 into a sampleionization section 276 in the second flow channel 272, with the negativeions flowing toward the positively biased electrode 275. These negativeions −,− ionize the sample S and carrier gas CG, and thus provide sampleions S−, S— for downstream analysis.

Preferably, as in FIG. 20F, DMS filter 277 and ion detector 278 areassociated with flow channel 272. The DMS analyzer benefits fromsplitting the ion flow, such that the sample is isolated from theharsher ionization chemistry that occurs directly within the plasmagenerator 11. In a further embodiment, the pressure in the first flowchannel 270 can be kept slightly lower (for example, and withoutlimitation, about 0.95 atm) than in the second (for example, and withoutlimitation, about 1 atm) so that a sweeping counter-flow keeps unwantedplasma ionization by-product from flowing into the second channel 272;this pressure gradient is indicated by the dotted arrows 279.

According to another feature of the invention, the plasma intensity canbe modified, modulated, and/or otherwise adjusted. In one illustrativeembodiment, the plasma and ionization chemistry occurring in the plasmagenerator is favorably modified by controlled introduction of a dopinggas D (FIG. 20F), for example, acetone. In this manner, plasma formationand ionization efficiencies can be increased as needed, which thenreduces the hysteretic effect of plasma ignition and burn. One result isthat the plasma ignition may be assisted, such as with a pulse ofdopant, and/or the ionization intensity may be sustained or operated ata higher level by continued flow of dopant and at lower power. Thus,high, middle, and low output intensities can be achieved through use ofsuch a control mechanism.

Use of the dopant for ignition reduces the energy required from thepower supply, reduces the destructiveness of ignition, reduces heatgeneration, simplifies circuit design and simplifies power sourcinglogistics, while sustained use of the dopant reduces the keep aliveenergy required to sustain the plasma. These contribute to lower powerdemand and reduced wear and longer life of the plasma source.

Another advantage of introduction of dopant D into the dual channelapparatus of FIG. 20F, is that the dopant accelerates and assists plasmaionization in the first flow channel 270 and then unwanted ionizationby-products are exhausted via the vent 280, while sample ionizationproceeds in the second flow channel 272.

Additional illustrative embodiments of the invention include arrays ofplasma sources, flow channels, analyzers, and the like, each of whichmay be operated under different conditions, such as with dopants D,excitation energies, plasma intensities, analytical conditions, and thelike, all to better control the analytical process.

Dopants may also be utilized in illustrative embodiments of theinvention according to the teachings of U.S. patent application Ser. No.10/462,206, filed Jun. 13, 2003, for METHOD AND APPARATUS FOR CONTROL OFMOBILITY-BASED ION SPECIES IDENTIFICATION, by Raanan A. Miller, et al.,the entire contents of which are incorporated herein by reference.

FIGS. 20G-20I show three illustrative arrays of the invention. In FIG.20G, the analytical array 200 includes a plurality of parallel flowpaths 12, each having a plasma ionization source 11 and DMS analyzer 20,the latter including a DMS filter 72 and detector 80. Sample, carriergas and dopant, if any, are delivered into the flow path for ionizationand analysis. The plasma generator is capable of delivering positive andnegative ions simultaneously, while each DMS is capable of analyzing thesame simultaneously and issuing an output indicating detection of bothion modes simultaneously. Therefore, having a parallel array accordingan embodiment of the invention enables a robust analytical effort to beamassed for a given task. Fast and reliable species identification isprovided.

Each flow path may appear as shown in FIG. 20G. An alternativeconfiguration is shown in FIG. 20H, where illustrative flow path 12 hasa first plasma ionizer 11 and a first analyzer. Dopant may be introducedinto the plasma generator 11 along with the carrier gas CG, or dopantmay be introduced along with the sample S into the less harshenvironment of the downstream sample ionization region 59. The ionizedsample is filtered 72 in the analyzer and detected at the detector 80,which provides a first set of data relating to sample identification.The sample is neutralized as it exits the detector 80, and therefore canbe re-ionized and reanalyzed and redetected under the same or differentconditions in the next in-line plasma ionizer 11, and the analyzer 20,as shown. A more robust analysis of the sample can be thus rendered.

In one additional embodiment, shown in FIG. 20I, the carrier gas CG isintroduced and ionized in the plasma generator 11 and the sample S isintroduced downstream and is ionized in the sample ionization region 59.The sample ions S+, S−, can be ionized in the analyzer 20 in the flowpath 12 a. However, the deflector electrode(s) 83 can be biased, asneeded, to deflect a positive or negative ion mode. For example, if theelectrode 83 in the flow path 12 a is biased negatively, then negativeions S− can be deflected into the flow path 12 b and further analyzed.The electrodes 83 may also be provided in flow path 12 b, and in thisexample, can be positively biased to attract the deflected ions. Thecarrier gas CG transports the ions S−, S− into the DMS analyzer 20accordingly. The remainder of the ion flow in the flow path 12 a can beprocessed or reprocessed downstream or can be vented, as desired, forthe particular analytical goal at hand.

In a further illustrative embodiment, the invention provides aconfiguration that reduces the complexity of chemical reactionsoccurring during the ionization process. As discussed above, the sampleS and carrier gas CG flow into the plasma 40 in the gap between theplasma source electrodes 14 and 16 and are thus ionized. Such ionizationcan produce a complex chemistry. As described earlier, if the gas isair, it can produce both positive and negative ions, usually including(H20)_(n), H⁺, O⁻, O₂ ⁻, O₃ ⁻, (N_(x)O_(n))⁺, and (N_(x)O_(y))⁻(H₂O)_(n)).

At times, such complexity can interfere with ionization and detection ofparticular analytes in the sample S. This can occur, for example, whenplasma formation and direct ionization of the sample in air includesformation of NO₂, which can interact with the sample and interfere withnegative mode sample detection. Therefore ionization of sample withoutformation of NO₂, and the like, is desirable in many cases.

FIG. 21 shows an illustrative embodiment of the invention that addressesthe challenges associated with complex chemistries by providing awindowless UV ionization source. Such a photon ionization source isdesirable for many applications and is shown in conjunction with anillustrative downstream DMS filter system.

More particularly, the analytical system 300 includes a plasmaionization section 301 and a DMS filter section 304, coupled by flowpath 306. Optionally, the system 300 also includes a downstream detectorsection 307. The flow path 306 includes a transport gas inlet 308 at thedownstream end of plasma source 302 and a further downstream sampleinlet 309 upstream from the DMS filter 310. The flow path 306 extendsbetween an upstream outlet 315 via a needle valve 317 and a meter 318above the plasma source 302 and a downstream outlet 319 the belowdetector section 307. The gas flow from the source 308 is split into anupstream flow 308A and a downstream flow 308B, which may be regulated bycontrol of the needle valve 317.

The transport gas is ionized to generate ions and photons in the plasmagenerated between the electrodes 314 and 316 of the plasma generator302. The ionized gas flow and its products is carried by the flow 308Ato the outlet 315 away from the filter section. However, the photons Pgenerated in the plasma section, travel downstream adjacent to thesample inlet 309, in a photo-ionization region 311, into the ion filtersection 304 between DMS ion filter electrodes 323 and 324. Uponfiltering, the ion species of interest are passed by the filter into thedetector 307. According to this illustrative embodiment, both positiveand negative ion modes of the passed species are detected substantiallyconcurrently/substantially simultaneously at the oppositely biaseddetector electrodes 325 and 326, with the expended flow then proceedingto the outlet 319.

Thus, in various illustrative embodiments, the invention provides ahighly functional windowless UV ionization system. This windowlessfunction avoids the degradation of the UV window. As has been observed,NO₂ can interfere with the detection and identification of ion species,either by obscuring detection of an analyte, causing peak shifts, or thelike. When the carrier gas is air, ionization of the carrier gas cangenerate NO₂. Another benefit of photon-based ionization of the sample Sand the carrier gas CG in the flow 308B in the photo-ionization section311 is that the production of NO₂, and its interaction with speciesformation is significantly reduced. The reduced NO₂ stream then flowsinto the analyzer 304.

According to the illustrative embodiment of FIG. 21, a first gas flow308A, which may be referred to as a counter-flow or curtain gas, isdirected into the plasma ionization section 302 to generate the plasma321. The ionization by-products are swept from section 302 by the flow308A away from the photo-ionization region 311. Thus, photons from theplasma enable photo-ionization of the sample S to proceed without NO₂interferences. This embodiment therefore provides an improved photonsource and an improved sample photo-ionization environment.

FIG. 22 shows a system 330 employing an alternative photo-ionizationembodiment of the invention. The system 330 performs similarly to thesystem 300, but with a different flow pattern. More specifically, thesystem 330 includes a plasma ionization section 301, a DMS filter 304,and a detector 307. An ionizable gas flow, such as the air flow 332,flows from the inlet 334 to the outlet port 336 between the electrodes314 and 316 and through the plasma 321 formed by plasma generator 302.This is at the upstream end of the flow path 333.

Ions and other by-products are generated in the plasma and are exhaustedfrom the flow at the outlet port 336. The exhausting may be assisted bya controlled pump 337. The photons P are also generated and flow intothe sample photo-ionization section 311 adjacent to the sample deliveryport 309. The soft ionization of the sample, the DMS species filtering,and the detection then proceed, as earlier described, with little or nounwanted effects from the plasma ionization process.

A portion of the gas inflow 332 splits into the flow path 333 and flowsdownstream, carrying the ionized sample from the photon-ionizationsection 311 and into the downstream analyzer's DMS filter 304. Thecarrier gas CG and sample S that are ionized and generate ions +,−,including ionized sample S−,S+, flow into the analyzer section 304 (DMSfilter), without generation of NO₂ and avoiding other processcomplexities. In one aspect, interaction of the photons P and the sampleS is at least partially responsible for producing the ionized samples S+and S−.

Thus, in these embodiments, a first gas flow is for formation of plasma,wherein the flow also sweeps unwanted ions and the like away from thephoto-ionization section. The photo-ionization of sample may be achievedusing a second separate gas flow, or a split flow as shown earlier, suchas where the pressure from the sample inlet side is higher than presentin the plasma ionization region 301. But in any event photo-ionizationtranspires in a controlled environment and avoids the formation of NO₂and the like, enabling improved downstream ion species analysis.

In a further embodiment of the invention, a strategy for controllingplasma level is provided. Open loop techniques can be used for thispurpose but make it difficult to accommodate non-linearities in plasmageneration. In a preferred embodiment, a feedback loop is formed toenable close regulation of the output of the plasma generator. Referringto FIG. 22, photo-detector 340 is placed adjacent to the plasma source321. Now, the level of emission or generation of photons within theplasma source can be detected and used as a measure of plasma output.Photons continue to flow into the photo-ionization section withoutinterference, while photons which otherwise would not travel into thephoto-ionization region are detected by photo-detector 321. Thisdetector connects to the system controller, such as controllers 22 d(FIG. 1C) or 222 (FIG. 20B), enabling detection and maintenance of theplasma/photon level by direct measurement. We have found that lightintensity is directly proportional to plasma/ionization level, andtherefore is a good performance measure.

In one embodiment, a conventional photo-detector, with response near theemission wavelength of 400 nm was used to measure the light intensity.As a result, the drive signal to the plasma generator was controlled toregulate the plasma and/or photon intensity at a desired steady state.This enabled ionization of the sample S to proceed under preferred andexpected conditions. In a further embodiment, a photo-spectrometer wasused to more completely tune the photon spectral output.

In a further embodiment of the invention, the filter electrodes 323 and324 are substantially simultaneously RF-driven and DC-biased and act asattractor electrodes, which draw ions from the plasma ionization source302 into the photo-ionization section 311 for increasing ionizationefficiency of the media thereat in the second gas flow.

In the embodiment of FIG. 22, the control electrodes 346 and 347 aredriven to attract or repel positive and/or negative ions and therefore,control introduction of such ions into the photo-ionization section foradditional control of the ionization process in the photo-ionizationregion 311. The plasma drive according to various illustrativeembodiments of the invention will now be discussed. Returning to FIGS.1A-1C, it will be appreciated that use of the resonant drive 22 forgenerating plasma at plasma source 11 for producing a high frequency RFplasma field decreases power consumption. In a preferred embodiment, theresonant drive circuit 22 also provides system stability via a feedbackloop. More particularly, it will be appreciated that plasma pumpingenergy strongly depends on field strength. If applied voltage and/orfrequency (and electric field correspondingly) rises, then dischargeenergy increases. This results in increasing rate of ionization and,consequently, an increase in discharge activity. Unchecked, thisincrease can result in a excessive increase in plasma energy andheating.

In one embodiment, the electrodes 14 and 16 act as a capacitor in L-Cresonant circuit 22 c. If the capacitor's conductivity increases, thenthe Q-factor of the resonant circuit decreases. Since applied voltage isproportional to the Q-factor, the voltage decreases as well. Thus, theelectrodes 14 and 16 are part of a negative feedback loop formaintaining the plasma at a desired energy level for a given drivevoltage and frequency, preventing runaway plasma growth and overheating.

A particular drive circuit 22 design depends on target plasma levels andelectrode and gap dimensions. Nevertheless, typically, an AC voltage,with an amplitude sufficient to produce a large electric field F, isrequired to initiate and maintain the discharge in atmosphericconditions. The reactive power in the megahertz frequency range is onthe order of tens of watts for a capacitive load of tens of picoFarads.Therefore, an illustrative embodiment of the invention employs aresonant oscillator, with a capacitive load as a component of the outputLC-circuit.

In further embodiments of the invention, modulation of the plasmagenerator drive signal reduces power consumption. Use of such modulationimproves spectrometer performance (whether such modulation is smallsignal, large signal, analog, digital or otherwise). This modulationencodes the ionized sample signal to be detected.

This characteristic modulation can be used to discriminate against noisecontribution that is spread over a different and wider band offrequencies. An AC coupled amplifier, with a narrow band filter centeredon the modulation frequency, significantly increases the signal to noiseratio of the spectrometer. Additionally, the filter frequency pass bandcan be narrowed to the limit of the modulator stability. Alternatively,if noise contribution is still too high or to negate modulator signaldrifts or shifts, lock-in techniques, such as actively tracking themodulator frequency and using a narrower filter band, can be employed.

Several benefits of ionization modulation can be obtained in practice ofthe invention. These benefits include, without limitation: improvedsignal to noise ratio, ionization of selected chemical species, reducedionization of interfering species, improved detector performance,optimization of system power use.

The invention is not limited to use of inert gases, as in some prior artsystems. The invention has improved configurations, with use of abroader array of gases, including air (whether filtered, ambient, orprocessed zero air), depending upon species being detected.

The invention, in various illustrative embodiments, also incorporatesthe ability to change plasma source intensity by control of how it isexcited, such as with a signal varying between 10's of kHz to 100's ofMHz. By way of example, in one low power embodiment of the invention,the plasma is driven at about 1 MHz. The plasma ionization source of theinvention may be driven using various drive circuits, whether LC,transformer, resonant, H-bridge, planar magnetic, or the like. It is afurther benefit of the invention that the plasma ionization source, invarious configurations, is designed to be integrated into and/or usedwith a mass spectrometer and/or ion mobility based analyzer, andpreferably a DMS filter/detector.

The plasma source of the invention has various applications, and apreferred embodiment is used in conjunction with a DMS analyzer. Anillustrative plasma drive circuit of the invention is shown in thecontext of a DMS chemical analyzer in FIG. 23. The system 400 generatesand analyzes ions according to the invention. The system 400 includes anionization source 405 and DMS spectrometer 408 (having a filter anddetector). The ionization source 405 ionizes fluids, such as gases, andthe spectrometer 408 filters the ionized gases and detects the gas ionsthat pass through the filter. The system 400 includes a flow path orchannel 104, which may be at atmospheric pressure, in which the ionstravel.

The ionization source 405 includes a plasma source interface 406, whichincludes first and second differential source electrodes 407 a and 407 b(collectively, source electrodes 407), and a Radio Frequency (RF) sourcedriver 420 connected to the source electrodes 407. In response to asufficient voltage being supplied across the source electrodes 407, adischarge field is established. A gas sample 440 is flowed into thefield between the source electrodes 407. The gas sample 440 is ionizedby capacitive discharge between the source electrodes 407. Thisdischarge ionization produces a plasma from the plasma source 435, withboth positive and negative ions.

In a preferred DMS spectrometer 408, forming these ions enables chemicalanalysis of compounds, where ion species are separated based ondifferences of mobility in a DMS filter field. Spectrometer 408 includesa filter 410 and detector 415. The filter 410 includes an ion interface411 and filter driver 425. The ion interface 411 includes filterelectrodes 412 a and 412 b. Similarly, the detector 415 includes afiltered ion interface 416 and detector processor 430, that latter whichmay be provided as an on-board component or as a separate device. Asshown, the filtered ion interface 416 includes detector electrodes 417 aand 417 b.

A gas plasma source 435 is ionized by the discharge in an RF fieldbetween the source electrodes 407. The plasma in turn ionizes the gassample 440 and forms ions 445 (e.g., M⁺, MH⁺, and M⁻). Generated ionsexit the ionization source 405 to the spectrometer 408 for analysis.

Driver electronics are provided to drive the electrodes of theionization source 405 and the filter 410. A source driver 420 isconnected to the source electrodes 407. A filter driver 425 is connectedto the filter electrodes 412 a and 412 b. In addition, detectorprocessing 430 may be applied to measure the ions 445 received by thedetector electrodes 417 that improves the performance of the detectionof the ions 445 based on known characteristics of the source driver 420.The characteristics may be provided by the source driver 420 to thedetector processing 430 via a bus 450.

The plasma source driver 420 enables plasma generation. Electronics usedto generate plasma are typically of high power, high cost, and lowreliability due to high frequency, high voltage, and complex loadpresented by the plasma source. In contrast, a preferred embodiment ofthe source driver 420, according to the principles of one aspect of theinvention, generates low energy plasma using low power (<about 1 W) andenables linear control of the plasma intensity enabling fine control ofthe ion 445 generation.

Controlling plasma intensity by varying applied voltage is difficult dueto the hysteretic and threshold nature of the plasma. FIG. 24 is a plotindicating the area of hysteresis. The plasma intensity at points A andB are more than necessary, if not too much, for the detector 415. Inother words, at the level of high optical intensity as shown on thecurve, there are too many ions for sensitive measurements to be madewhen fine measurements are desired. A non-modulated voltage produces thesharp curves as shown in FIG. 24, which includes the hysteretic bandbetween points A and B. Thus, low optical intensities cannot be achievedthrough use of a source driver that provides voltages in a mannerproducing such curves.

In contrast, FIG. 25 provides a curve resulting from the source driver420 producing a duty cycle modulation on the drive signal. By duty cyclemodulating a switching waveform applied to the plasma source 435, linearcontrol of the intensity of ions produced can be achieved. In an exampleionization source, a 3% duty cycle as shown in FIG. 25 may be optimumfor generating plasma. In an example DMS system 408, this enablesreliable and sensitive measurements at detector 415.

In addition to allowing linear control of the ion production, modulationinformation associated with the source driver 420 can be used to improvesignal-to-noise ratio of the recovered signal by the detector 415. Theapplied modulation, shown in FIGS. 26A-26C, results in the ions 445being produced at the modulation frequency (e.g., 2 kHz). The ions 445may be interchangeably referred to as ion packets 445 hereafter. Knowingthis frequency and allowing for a measurable phase delay down the lengthof the channel 104 of the system 400 allows for the use of synchronousdetection of the ion packets 445. The modulation frequency may beprovided to the detector processor 430 via the bus 450. This results inlarge improvements in the signal-to-noise ratio as a result of reducingsensitivity of the detector 415 to signals of any other frequency andphase.

The modulation itself may be designed to operate a transformer (notshown) employed by the ionization source 405 at resonance (e.g., 2 MHz)with a peak-to-peak voltage of sufficient amount to produce the ionpackets 445, such as a signal level of 2500 Vpp. By providing themodulated signal at a given duty cycle, such as about 3%, plasmaintensity sufficient to produce ion levels compatible with the detector415 can be achieved, which in one example is at about 100 mW.

Other benefits beyond the savings in power may also result fromoperating at lower energy states. For example, the ionization source405, when operated in accordance with the intensity curves of 23A,generates a relatively high amount of ozone, which may interfere withsensing of the ion packets 445. Moreover, the ion packets 445 of highenergy level can cause breakdown, caused by thermal effects andablation, of uncoated electrode plates 407 a and 407 b and 412 a and 412b, or isolation materials used to protect the electrodes. For example,in some previous designs, the electrodes and/or isolation materials havea typical lifetime of about 100 days. With this new, lower energytechnique, the electrodes and/or isolation materials may lastindefinitely. In addition, the heating of the electrodes 407 in theionization source 405 is caused by the high energy states of the plasmasource 435 in previous techniques; however, with this new, low energytechnique, heating does not occur to significant levels and heat sinksare not necessary, which allows the system 400 to have a smaller overalldesign, lighter weight, and be better suited for a hand-held design.

Referring now to FIG. 23 and FIG. 28, the ionization source 405 producesions in ion packets 445, which are filtered by filter 410 such as by DMStechniques. In one embodiment, the ion packets 445 passed by filter 410are received at detector electrometer plates 417. One of the detectorelectrometer plates 417 a is positively charged with respect to ground,repels positively charged ions in the ion packets 445, and attractsnegatively charged ions in the ion packets 445. The other of thedetector electrometer plates 417 b is negatively charged with respect toground, repels negatively charged ions in the ion packets 445, andattracts positively charged ions in the ion packets 445. The DC offset515 from ground is applied to the two detector electrometer plates 417,although only plate 417 b is shown in FIG. 28.

Detection of the ions 445 at the detector electrometer plates 417results in a small (e.g., pico amps, pA) amount of current that is thenamplified by, for example, a transimpedance amplifier 505. Thetransimpedance amplifier 505 may include, for example, a feedbackresistance element of tens to hundreds of mega ohms (Mohms). Thus, thesmall current is amplified to a voltage range that can be interpreted bya processor 510, such as a digital signal processor (DSP).

In various illustrative embodiments, the plasma field has an RFcomponent that may be of a standard or custom shape (e.g., sinusoidal,bias offset, pulse width modulated, or otherwise). For example,embodiments of the invention are operable with a sinusoidal highfrequency high voltage waveform applied to plasma electrodes 14 and 16,as shown in FIG. 26A, modulated as in FIG. 26B or pulsed (“packetized”)as in FIG. 26C.

Use of the packet waveform increases discharge stability, decreasespower consumption, and further controls ionization efficiency. Morespecifically, the pulsed design follows from the recognition that afinite time interval is required for the plasma instability to reach themacrolevel. Therefore, energy is delivered to the discharge gap by shorthigh frequency (RF) high voltage high intensity pulses, so that theinstability does not have the time to develop. Once a pulse is switchedoff, dissipative processes suppress the development of the instability.If the pulse repetition period is comparable to the energy relaxationtime in the plasma, its period-averaged parameters, including the degreeof ionization, will be quasi stable. In one illustrative embodiment, thepulse had a frequency of about 1-20 MHz, a duration of about 1 msec, anda peak-to-peak voltage of about 1000-10000 volts. In one illustration,the duty cycle (t₁/t₂) of the packet waveform was approximately 1/11.

Use of the packet waveform is beneficial. Because the efficiency ofionization of the plasma ionization device 11 is directly proportionalto the voltage supply duty cycle, drive circuit 22 consumes less power(proportional to duty cycle) to provide the pulsed waveform versus thecontinuous waveform. Further, the service lifetime of the ionizationdevice 11 increases by a factor of 5 to 10 times when the ionizationdevice is powered with a pulsed packet waveform.

With either continuous or packet waveform, a sufficient RF voltage willbe developed across electrodes 14 and 16 to cause the local gas toelectrically discharge and form a plasma. An advantage of operating thesource driver 420 (see FIG. 23) with a pulse width modulated signal isthe generation of ion packets 445 as opposed to a continuous stream ofions. This “packetized” transfer of ions 445 allows for the detectorprocessor 430 to use, for example, a synchronous detection technique asapplied to packets of filtered ions that arrive at the detector. Asynchronous detection technique can measure at a given frequency andphase to yield a high-Q, narrow band filter at, for example, 2 kHz,which is the modulation frequency used at the source driver 420 andprovided to the detector processor 430 via the local bus 450. Throughuse of this synchronous phase detection filter technique, such as in asynchronous filter, the signal-to-noise ratio of ion detection can beimproved between about 40 dB and 60 dB. Other processing techniques tofurther improve the gain or provide other advantages can be employedthrough the use of other DSP analysis or analog techniques.

FIG. 29 is a flow diagram of an example process for execution in the DSP510 for detecting the ion packets 445 just discussed. The process 600begins (step 605) when the detector processor 430 is turned on. Afilter, such as provided with the above synchronous phase detectionfilter technique discussed above, is initialized (step 610) based onpredetermined parameters. The sampled amplified detection of themodulated ion signal 620 is filtered (step 615), and the DSP 510 mayconvert and/or output the results (step 625) to a display or in acommunication signal to another processor, such as a data collection ormanagerial-type processor. The results may also be used as feedback to acircuit that controls the generation of the plasma; for example, theresults may be used to control the duty cycle applied to the plasma toincrease or decrease the amount of ions 445 generated by the ionizationsource 445 for improved and/or dynamic compensation of same.

Because of the packetized nature of the ion production, otherinformation may also be gleaned through use of this technique. Forexample, in the embodiment 700 of FIG. 30, a waveform of the drivesignal 705 and detection signal 710 can be compared against one anotherin the DSP on the rising edge or falling edge to determine a “time offlight” (TOF) of the ions from the ionization source 405 to the detector415, which provides useful detection information like that obtained inknown TOF ion mobility spectrometers. A DMS filter drive according to anillustrative embodiment of the invention will now be discussed. There isa need for efficient DMS filter drive circuit designs as may bedesirable for faster and more reliable operation, especially in thecontext of hand-held and battery-operated applications. Moreover,traditional designs typically exhibit poor efficiency due to the effectsof both parasitic and load capacitances. As discussed below, theinvention, in various illustrative embodiments, overcomes thesedeficiencies.

In the illustrative embodiment of FIG. 23, the filter driver 425provides excitation to the electrodes 412 a and 412 b of the DMS filter410. The excitation includes RF voltage and compensation. A preferredDMS system of the invention is driven as taught in U.S. ProvisionalPatent Application Ser. No. 60/498,093, incorporated herein byreference, wherein filter driver 425 uses a low duty cycle (e.g., lessthan or equal to about 20%), high voltage (greater than or equal toabout 1500 volts to peak), and high frequency (e.g., greater than orequal to about 1 MHz) signal applicable to ion filtering.

This results in power consumption less than or equal to about 1 watt. Bycomparison, a common DMS filter driver applies an RF voltage having 1500Vpp to the filter electrodes 412 and which in one embodiment translatesinto about 13 Watts of power required to operate system 400. At 13 Wattsof power, the system 400 uses more power than desirable for long periodsof hand-held usage and raises heat dissipation issues.

In a low power practice of the invention, the filter driver 425 takesadvantage of parasitic capacitances in transformers within the circuitthrough the use of a resonant reset technique to generate a high voltagepulse during the fly-back cycle. In addition to this technique, aspecially designed low capacitance planar transformer may be used todrive the capacitive load (i.e., filter electrodes 412) of the DMSfilter 410.

Advantages provided through use of this design include, for example:operating the circuit from a low voltage DC (e.g., 20 volts) source;employing a small geometry, low voltage MOSFET transistor to cause thefly-back switching of the transformer(s); using low cost components;sensing voltages on the low voltage primary side of the transformer(s)such that no high voltage resistors or capacitors are needed; achievingvery efficient (< or = about 1 W) in power consumption; and tuning thecircuit for operation at different frequencies. In one DMS embodiment,about 1500 volt operation at about 800 KHz using only about 750 mW isachieved, whereas traditional circuit types use >about 10 W.

FIG. 31 is a generalized schematic diagram for a filter driver 425according to an illustrative embodiment of the invention. In thisembodiment, four transformers 805 are arranged to provide power to thefilter electrodes 412 a and 412 b. Primary transformer windings 810 a ofthe transformers 805 are connected together in a parallel arrangement.One end of the primary transformer windings 810 a is connected to Vcc,and the other end of the primary transformer windings 810 a is connectedto a power return 825 through a MOSFET transistor 815.

Secondary transformer windings 810 b of the transformers 805 areconnected together in a series arrangement. At a location 827 betweenthe secondary transformer windings 810 b, a set of inputs 140 isprovided to present a DC offset to the filter electrodes 412 via thesecondary windings 810 b. A capacitor 830 is included to facilitateapplication of the DC offset.

FIG. 32 illustrates the leakage inductances and parasitic capacitancesassociated with the transformers 805 to aid in understanding theoperation of the filter driver 425 when used in the fly-backconfiguration of FIG. 31. Referring to FIG. 32, a parasitic capacitor(Cp) 905 spans between the primary transformer windings 810 a andsecondary transformer windings 810 b. A leakage inductor 910 isconnected in series between the secondary windings 810 b and a loadresistance 822.

The value of the leakage inductance 910 is proportional to the square ofthe turns. In operation, the leakage inductance 910 causes a voltagedrop when power is transferred from the secondary transformer windings810 b to the load resistance 822. In previous designs that used only asingle transformer 805, fewer primary turns are used to minimize thevalue of the leakage inductance 910, thereby minimizing voltage dropacross it. The problem is that fewer primary turns results in morecurrent experienced in the primary transformer windings 810 a and in thedrain-to-source path in the MOSFET 815. Because of the higher current,higher current rated elements, such as inductor wire and MOSFET devices,must be employed. However, through use of the parallel arrangement ofthe primary windings 810 a and series arrangement of the secondarytransformer windings 810 b, as shown in FIG. 31, a lower amount ofleakage inductance 910 is experienced.

In other words, in the traditional filter driver 425, a singletransformer results in a dramatic increase of leakage inductance 910,such as by a factor of 16. Meanwhile, because of the parallel/seriesarrangement of the primary transformer windings 810 a and secondarytransformer windings 810 b, respectively, the increase in leakageinductance 910 is considerably less, such as only increased by a factorof four with four transformers 805 to produce the same output voltage asthe prior, single transformer design.

Moreover, in typical applications, the voltage gain across a transformeris generally desired to be less than or equal to about 4:1, whereV₀/V_(in) is proportional to N_(secondary). In the ion filteringapplication at hand where a pair of filter electrodes 412 are beingdriven, the voltage gain is approximately 16:1. This gain is consistentwith the parallel/series designed discussed above.

The parasitic capacitance 905 is used to further increase thetransformer gain in that the MOSFET 815 can be switched at a rate thatmatches the oscillation frequency experienced between the primarytransformer windings 810 a and parasitic capacitance 905. In this way,the voltage differential between VCC 820 and power return can be madelower (e.g., 20V rather than 200V) and still produce the necessaryoutput voltage for driving the filter electrodes 412 at a levelsustaining proper operation, i.e., filtering of the ion packets system445 (FIG. 23).

FIG. 33 is an example RF waveform 1005 used to drive the filterelectrodes 412. The waveform 1005 has a duty cycle of approximately 20%on and operates at a frequency of about 1.2 MHz, with peak-to-peakvoltages of approximately 1500 Vpp. This waveform tracks an input signal830 (FIG. 31) provided to the gate of the MOSFET 815, where the waveform1005 is shown in FIG. 31 as a differential drive signal 835 a and 835 bpresented to respective filter electrodes 412 a and 412 b.

FIG. 34 depicts a waveform (e.g., sawtooth waveform) 1010 that is usedto provide the compensation voltage applied to the DC offset inputlocation 827 (FIG. 31) in the secondary transformer windings 810 b. Thewaveform 1010, in this embodiment, has a peak positive voltage of about+15V, a peak negative voltage of about −45V, and a user selectablefrequency of, for example, about one scan about every four to twentyseconds, optionally corresponding to a user display. Such a waveform isdesigned to properly filter the ions in the ion packets system 445,produced by the ionization source 405, traveling through the filter 410to the detector 415. Based on the types of ions traveling through thefilter 410, which is a function of the plasma source 435 and sample 140under test, the compensation voltage waveform 1010 may be changed inshape, frequency, and/or voltage range. A feedback arrangement may beprovided, where the detector processor 430 may have some optimizationprocesses that provide feedback to the filter driver 425 toautomatically adjust the characteristics of the compensation voltagewaveform 1010.

FIG. 35 is a graphical diagram of the ion packet system 445 travelingbetween the filter electrodes 412 a. The example compensation voltagewaveform 1010 is applied to the filter electrodes 412, causing the ionpackets system 445 to oscillate between the plates. Ions system 445 nothaving characteristics compatible with the compensation voltage waveform1010 “drop-off” from the ion packets system 445. The ion system 445 thatpass through the filter electrodes 412 contact the collector plate 417b.

Beyond the circuit improvements discussed above, improvements to thetransformers themselves may also be used to improve overall filterdriver 425 performance.

FIG. 36A is an example of one such improvement, where a top view of ahelical shape is shown. The helical shape provides the primary andsecondary transformer windings 810 a and 810 b of the transformers 805.A first connection point 1205 a may be connected to the voltage sourceVCC 820, and a secondary connection point 1205 b may be connected to theMOSFET 815, as discussed in reference to the primary transformerwindings 810 a of FIG. 31.

Continuing to refer to FIG. 36A, the helical windings may be implementedon a printed circuit (PC) board that inherently has a high withstandvoltage (i.e., arcing is prevented for up to very high voltages). Thehigh withstand voltage transformer is an improvement over traditionaltoroidally wound transformers that use transformer wire that has a lowerwithstand voltage for a similar cost of materials. In addition, theprinted circuit board has a smaller physical size than toroidally woundtransformers and can be made through less expensive automatic productiontechniques.

Referring to FIG. 36B, a cross-sectional view of a six-layer printedcircuit board 1210 of the helical shape of the windings 810 a, 810 b isshown. The printed circuit board 1210 includes layers 1215 a, 1215 b, .. . , 1215 f with traces 1220 used to implement the helical design ofFIG. 36A. The primary transformer windings 810 a are printed on theoutside layers 1215 a and 1215 f in this embodiment, and the secondarytransformer windings 810 b are printed on the two innermost layers 1215c and 1215 d. The layers without the traces 1220 (i.e., layers 1215 band 1215 e) provide layers where the center point 1203 of the helicaldesign (FIG. 36A) is routed from the center to the connection point 1205b. A full circuit board 1210 supports a single or multiple transformers,depending on the size of the circuit board 1210. For example in oneimplementation, a 1″×1″ square circuit board supports a singletransformer 805.

With the topology of FIG. 36B, a build-up of capacitance (i.e.,capacitance is proportional to the trace area divided by the distancebetween the traces) is incurred, which causes inefficiency in theoperation of the transformers 805. So, in order to reduce thiscapacitance, a different layout may be employed, shown in FIG. 37.

FIG. 37 is a top view of a layout of the primary transformer windings810 a and secondary transformer windings 810 b. The windings 810 areconcentric spirals, where, in this embodiment, the primary transformerwindings 810 a are “within” the secondary transformer windings 810 b.Mechanically, the traces 1220 on a 6-layer circuit board 1210 may be ondifferent layers 1215 a, 1215 f; 1215 c, 1215 d, and “via” layers 1215b, 1215 e may support extension of the center points 1203 and 1303, tothe respective connection points 1205 b and 1305 b. Traces to the otherconnection points 1205 a and 1305 a may be on one of the via layers orwindings layers.

In operation, coupling between the outer concentric winding (e.g.,secondary transformer winding 810 b) and a center magnetic core(discussed later in reference to FIG. 38) is reduced due to an increaseddistance between the windings and the center magnetic core. However,this loss is less than increases resulting from offsetting the traces1220 composing the primary and secondary windings 810 a and 810 b, whichreduces capacitance therebetween.

FIG. 38 is an example of the 6-layer circuit board 1300 b of FIG. 36Bthat is sandwiched between a magnet having first and second components1405 a and 1405 b. These components 1405 have a central peg, which maybe about ¼″ in diameter. The central peg 1410 feeds through the circuitboard 1300 b, and, consequently, in the center of the helicaltransformer windings 810 a and 810 b, as just described, to form atransformer adapted to meet the performance metrics discussed above.

While this invention has been particularly shown and described withreference to several embodiments, it should be understood that variouschanges in form and details may be made herein without departing fromthe scope of the invention encompassed by the appended claims.

As can be seen from the above discussion, plasma sources according tothe invention are useful in a wide range of systems that require sampleionization. The invention may be provided as a stand-alone device or maybe incorporated into a larger system that can benefit from a clean andstable source of ions. Examples of such systems include DMS systems, ionmobility spectrometers, and atmospheric chemical pressure ionizationspectrometers, among others. However, practices of the invention are notlimited to analytical purposes, and in fact, the invention has manypractical applications too numerous to list herein.

1. (canceled)
 2. A compact differential mobility spectrometer (DMS)analyzer comprising: an ionization source, a chip-based DMS filter forpassing select ions from the ionization source through an analyticalgap, the analytical gap being defined by at least one pair of filterselectrodes, a driver circuit for providing a filter signal to the atleast one pair of filter electrodes, the driver circuit including avoltage combiner for combining a time-varying signal with and a DCsignal to form the filter signal.
 3. The analyzer of claim 2, wherein aportion of the voltage combiner is included on a printed circuit board.4. The analyzer of claim 3, wherein the voltage combiner includes anintegrated circuit amplifier.
 5. The analyzer of claim 4, wherein theintegrated circuit amplifier includes a MOSFET.
 6. The analyzer of claim4, wherein the DC signal originates from a DC voltage source and thetime-varying signal originates from a time-varying voltage source. 7.The analyzer of claim 6, wherein the chip-based DMS filter and thedriver circuit are included in an integrated package.
 8. The analyzer ofclaim 4, wherein the voltage combiner includes at least one transformer.9. The analyzer of claim 8, wherein the at least one transformerincludes primary windings and secondary windings.
 10. The analyzer ofclaim 9, wherein at least one of the primary winding and secondarywindings are embedded on a circuit board.
 11. The analyzer of claim 10,wherein the DC voltage source is in electrical communication with thesecondary windings.
 12. A method for analyzing a sample comprising:ionizing a portion of the sample, passing through select sample ionsusing a chip-based ion mobility based filter, the ion mobility basedfilter including at least one pair of filters electrodes, combining atime-varying signal with and a DC signal using a voltage combiner toform a filter signal, and providing the filter signal to the at leastone pair of filter electrodes.
 13. The method of claim 12 comprisingincluding a portion of the voltage combiner on a printed circuit board.14. The method of claim 13, wherein the voltage combiner includes anintegrated circuit amplifier.
 15. The method of claim 14, wherein theintegrated circuit amplifier includes a MOSFET.
 16. The method of claim14, wherein the DC signal originates from a DC voltage source and thetime-varying signal originates from a time-varying voltage source. 17.The method of claim 16 comprising including the chip-based ion mobilitybased filter and the driver circuit in an integrated package.
 18. Themethod of claim 14, wherein the voltage combiner includes at least onetransformer.
 19. The method of claim 18, wherein the at least onetransformer includes primary windings and secondary windings.
 20. Themethod of claim 19, wherein at least one of the primary winding andsecondary windings are embedded on a circuit board.
 21. The method ofclaim 20, wherein the DC voltage source is in electrical communicationwith the secondary windings.